Fuse holder



J 28, 1953 c. c. SHEPHERD, JR 2,821,603

FUSE HOLDER Filed Dec. 23, 1955 2 Sheets-Sheet 1 FIG. 3

INVEN TOR.

CLAYTON 0. SHEPHERD,JR.

Y flaw/5% ATTORNEY Jan. 28, 1958 C. C. SHEPHERD, JR

FUSE HOLDER 2 Sheets-Sheet 2 Filed Dec. 23, 1955 IN VEN TOR. CLAYTON G. SHEPHERD, JR.

ATTORNEY United States Patent FUSE HOLDER i Clayton C. Shepherd, Jr., Downey, Calif., assignor to NorthAmerican Aviation, 'Inc.

The present invention concerns a fuse holder fora chemically etchable fuse link. More particularly, this invention is directed to a holder for a fuse link used during the etching of a workpiece to monitor the attainment of a predetermined depth of etch in that workpiece. A distinct problem in the chemical etching of metals is to determine when a predetermined amount of etching of a workpiece has been accomplished without continually removing the workpiece from the etching bath for a visual or mechanical check of the etched depth. This has been accomplished by monitoring the attainment of such depth by providing a fuse link simultaneously exposed to the sarne etching action as the workpiece. The fuse links are ordinarily of the alloy or metal being etched and have a critical thickness equal to the desired depth of etch. When the fuse link is etched away, a circuit is completed or broken in an electrical monitor which, in turn, may actuate a visual or audio signal or a device removing the workpiece from etching action at that predetermined depth of etch.

. Heretofore, the fuse links adapted for immersion in the etching bath were held by a bolt and wing-nut combination to which the electrical monitor wires were connected. Very erractic results were obtained from the fuses so held. The fuses were etched away under the wing-nuts causing poor electrical contacts, and, in the case of etching aluminum, aluminate and other by-prod ucts build up on the fuse surfaces thus breaking a cir The monitor seen in Fig. I

quit through the fuse. provides for current flow through the fuse link and senses an electrical discontinuity across the fuse link when the shuntetfect of the fuse is lost. It was also found that the fuse links were susceptible to being broken prematurely by turbulence in the bath or, conversely, having broken etched-through ends stick together, thus not breaking the circuit through the fuse link. g

. The presently disclosed fuse holder seals the electrical contact areas of the fuse to avoid increase in electrical resistance due to the action of the etchant. It also maintains firm positive electrical contact of the fuse and the contact assembly and supports the fuse in such a manner that the area to be etched is in good contact with the etchant at alltimes. and removed from the present device and, by reason of holdingthe fuse rigid, premature breaking and the possibility of the fuse ends coming together after the center is etched away is prevented.

An object of this invention is to provide a holder suit able for holding a fuse link subjected to etching action.

A further object of this invention is to provide a holder 1 in combination with a fuse link adapted to monitor a.

predetermined depth of etch in a workpiece. I

A still further object of this invention is to provide a iuselinlt holder of simple construction insuring positive Fuse links are easily mounted Patented Jan. 28, 1958 electrical contact between an inserted fuse link and terminals on said holder.

An additional object of this invention is to provide a clamp type holder for an etchable fuse link. I

The above objects as well as other objects of this invention will become apparent from the following drawings inwhich Fig. 1 shows the overall use of the fuse link holder of this invention; I

. Fig. 2 shows a partial cutaway of the holder; and

Fig. 3 is a modificationof the holder.

Fig. 1 schematically illustrates the use of a separate fuse link to monitor thedesired etching depth. Etching of various workpieces is normally carried out in a tank 1 containing an etchant chemical. The particular etchant chemical used depends on the metal being etched and on the desired etching rate. In the etching of aluminum, for example, caustic soda is normally used. The workpiece 2, as shown, comprises a curved sheet having an etch proof film 3 on the edges thereof. The action of the etchant etches away the metal in the area 4, resulting in a chemically milled sheet having a central area of less thickness than the original material and the remaining nonetched portions. Handling equipment 5 is normally present to move the sheet 2 in and out of the tank 1 as desired. Due to the consumption of etchant material, change in bath conductivity and temperature and to the formation of various by-products in the etchingprocess, the etching rate is not constant throughout the etching cycle. Thus, a control on a straight-time basis is inapplicable to most etching processes. The difliculties inherent in removing the sheet 2,, or many sheets in the same bath, for actual physical measurement of thecentral thickness is apparent. y

As illustrated in Fig. 1, a fuse link 6 is placed within the tank 1 and is exposed to the same etching conditions as the workpiece 2. The fuse link '6 is made of the same material as the workpiece and is etched at the same rate as the workpiece. The fuse link'6 has at least one minimum dimension which is proportional to the desired depth of etch. In the normal fuse link, this minimum dimension is such that an effective electrical or. mechanical discontinuity occurs in the fuse link when the predetermined depth of etch has been determined- In the case of a separate fuse link, the minimum dimensionv will ordinarily be equal to the desired depth of etch. In sensing an electrical discontinuity, the hereinafter described shunt elfect of the fuse link ends at a critical value of approximately one microinch or less, dependent on the actual fuse dimensions. This value is thus, they theoreti cal error. Howeverydue to the relatively greater error inherent from the etching attack, tolerances in the' herein described monitoring means will ordinarily, be ,of the. order of plus or minus oneor two mils.- Duringhthe simultaneous etching attack on the workpiece 2 and the: fuse link 6, an electrical current is, passed through the fuse link 6 through leads 7 and 8.. A sensing unit 9,.pref-' erably in an enclosed unit in close proximity to the tank 1, is capable of sensing a discontinuity in the fuse'li'nk 6- When this discontinuity occurs and is sensed, a sensing signal may be sent to a triggering and signaling means 10 by leads '11, or to another triggering means 12 and "to the handling equipment 5 by leads 13. Thus,"the open ator of the etching bath will be appraised of adisc'onti unity and the attainment of the predetermined etching depth by a visual or audio signal from the devicef10 and/er the workpiece may be automatically removed from,

the bath. The details of the sensing unit and triggering means, not forming any part of the present invention, may be seen in United States patent application Serial No. 453,755, filed September 2, 1954, assigned to the assignee of this application.

The fuse holder combination of this invention shown in Fig. 2 comprises a base structure including a base 21, preferably of electrical insulating material, and an electrically conductive disc 24 forming part of the fuse contact assembly. Actual electrical contact between the fuse link 23 and the conductor disc 24 is effected through contact pins 20 extending from the surface of the disc 24. A resilient disc 27 of a chemically etched-proof material, such as neoprene, is provided with apertures which are adapted to receive the contact pins 20. The uncompressed thickness of the resilient means 27 is greater than the length of the extending contact points insuring a positive contact andseal of the contact points when the fuse link 23 bridges the conductive members 24 and is forced down onto the rigid pins 20. The clamping means of this modification includes a clamping yoke 22 attached to the base 21 and positioned to hold a bolt means 28. The bolt means 28 is attached to a ciamping disc or bar 25 which is adapted for downward movement upon turning of the handle 29 of the bolt 28. A plastic disc 26 is attached to the disc 25 to provide an aditional seal at the inner face of the fuse link and the clamping bar 25. Such seal is particularly important when the fuse link does not contain the channel as shown. Terminal means 19 are provided on each of the discs 24 for attaching lead wires to the monitoring apparatus. As shown, an electrical discontinuity will occur in the fuse link 23 when the thickness in the channel portion of the fuse link 23 is etched through. This thickness will vary according to the etchant depth desired in the workpiece being simultaneously etched with the fuse link in the etching bath.

In the Fig. 3 modification, firm positive electrical contact with the fuse is provided by employing a spring loaded needle point contact within the holder. The base structure 31 of this modification is provided with bore portions into which are fitted insulating sleeves 39 and insulating discs 32. The point of contact 33 extends through a bore concentric with the first-mentioned bore and extends further through an aperture in an insulating disc 37. A spring 30 is provided in the bore which abuts the head of the pin 33 and a rear closure part 34. EX- tending from the closure 31 is a terminal means 19 for connecting the lead ends to the aforementioned depth limit-monitor. A single yoke portion 35-is attached to the base portion 31 and has acam 38 suitably attached thereto. n manualmovement of the cam 38 a clamping bar 36 havinga neoprene or other sealing material disc 40-attached thereto is'forced downward upon the fuse link 23 which has been inserted in the space between the pads '40 and 37, against the positive spring pressure of thecontact points 33. When the bar'36 is in the full dowmposition-a seal is effected around the contact points 33 'due'to thercooperation of theresilient pads 37 and 40.

Various means must be provided for insulating the base pairsofzcontact points from each other. The base portion31 maybe entirely insulating, as in the Fig. 2 modificationyorlitmay have an insulating insert separating eachshalf of the holder. Theentire unit as illustrated, save for the'fuselink itself, is preferably coated with -a'neopreneor'other'etchant-resistant material to be electrically insulated from the etchant solution. This surfaceinsulation reduces the amount of current being conducted through the 'etchant, increases the current through the fuse, and. increases the. :overallsensitivity of thefuseholder. Insulation of the base portion 31 may also :beseffected byproviding an insulative pad 45 and insulative insert 46 between portions of that base portion.

, Although the .invention has .-been described and .illustrated in detaiL'it is tobe clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. A fuse link holder combination adapted to be used in a corrosive fluid comprising a base structure, at least two electrically conductive contact points spaced apart and extending from said structure, a removable chemically etchable fuse link adapted in contact with said points, clamping means for clamping said link against said points and sealing means insulating said spaced contact points from corrosive action in the fluid.

2. The invention as set out in claim 1 in which means is provided cooperating with said contact points to insure positive contact of said points and said fuse.

3. A fuse link holder in combination with a fuse link adapted to be inserted in an etching bath comprising a base structure, at least two electrically conductive contact ponts spaced apart and extending from said structure, a removable chemically etchable fuse link in contact with said points, clamping means including a yoke at-v tached to said structure for clamping said link against said points, means insulating said spaced contact points and sealing means to seal the electrical contact area of said link and said points from etching action in said bath.

4. The invention as set out in claim 3 in which terminal means are provided in contact with said points and are adapted for passing a current through said fuse link.

5. A fuse link holder combination comprising an insulative base, spaced electrically conductive members attached to said base, terminal means on said members, at least one electrically conductive contact pin extending from each of said members, a compressible sealing means surrounding said contact pins and having an uncompressed thickness greater than the extending length of said contact pins, a removable fuse link bridging across said members and said sealing means, and clamping means, including a yoke attached to said base and at least one clamping bar, to clamp said fuse link into electrical contact with said contact pins.

6. A fuse link holder combination comprising a base portion, spaced bores extending within said portion, at least one movable contact pin means extending from one end of each of said spaced bores, a closure at the other ends of said bores, a spring in each of said bores abutting said contact pin means, a sealing disc around the extending contact pin means, a removable fuse link bridging across said contact pin means, clamping means, including a yoke attached to said base portion, for clamping said fuse link into electrical contact with said contact pin means, and means insulating said spaced contact pin Lneans in one bore from the contact pin means in another ore.

7. A fuse link holder to be used in a corrosive fluid comprising a base structure, at least two electrically conductive contact pins spaced apart a predetermined distance and extending from said structure, resilient means surrounding each of said pins, said resilient means adapted to cooperate with the ends of a fuse link bridged between said contact pins to seal the electrical contact area of the fuse ends and the contact pins from the cor rosive fluid, and means adapted to clamp the ends of the fuse link against said contact pins and resilient means.

8. A fuse link holder adapted to be inserted in a corrosive fluid comprising a base structure, at least two electrically conductive contact pins spaced apart and extending from said structure, a compressible sealing means surrounding said contact pins and having an uncompressed thickness greater than the extending length of said contact pins, and clamping means, including a yoke attached to said base and at least one clamping bar adapted to clan1 p the ends of a corrodable fuse link into contact with said contact pins and compressible sealing means.

9. A fuse link holder comprising a base portion, spaced bores extending within said portion, at least one movable contact pin means extending from one end of each of said space bores, a closure at the other ends of said bores, a spring in each of said bores abutting said contact pin means, a sealing disk around the extending contact pin means and clamping means adapted to clamp a fuse link extending between said contact pins against said contact pins and sealing disk.

References Cited in the file of this patent UNITED STATES PATENTS 866,090 Troutman Sept. 17, 1907 FOREIGN PATENTS 3,863 Great Britain Feb. 18, 1903 27,380 Great Britain Nov. 22, 1897 309,304 Italy July 3, 1933 

